A TDI function operates on the premise that two or more radiation detector channels, which are disposed colinearly in a scan direction and which view the same part of a scene during a given scan, will exhibit relative signal levels that are correlated, although separated in time. However, the noise associated with each channel is not correlated. By temporal phasing (a time delay function) and adding the radiation detector channel signals (an integration function) the resultant output signal has been found to exhibit a signal-to-noise ratio that is superior to that of a single detector channel. This is because the signals sum, but the noise is root sum squared, resulting in a signal-to-noise ratio improvement equal to the square root of the number of detectors.
By example, it is assumed that a bright "spot" moves, due to the scanning process, from a detector A to a detector B. The spot covers detector A at some time T.sub.1 and a sample of detector A is taken at that time. At some later time, T.sub.1 +t, the spot covers detector B and another sample is taken. By adding the sample from detector A, obtained at time T.sub.1, to the sample from detector B, obtained at time T.sub.1 +t, a higher signal-to-noise ratio estimate of the intensity of the spot is obtained.
Many conventional imaging systems implement the TDI function by causing all of the detectors to simultaneously take a sample of what the detectors are "seeing". This is often accomplished by sampling the detector circuitry through a switch that is coupled to a hold capacitor. When the switch is closed the voltage of the signal is placed on the capacitor. When the switch is opened this voltage remains on the capacitor until the switch is once more closed.
Conventional approaches for implementing the TDI function typically employ charge coupled devices (CCDs) or bucket-brigade device (BBD) circuits. However, CCD circuits require different and/or additional processing than does conventional MOS device fabrication. In addition, both types of conventional circuits suffer from transfer inefficiencies when the number of detector channels in TDI, that is samples per pitch, becomes large.
Commonly assigned U.S. Pat. No. 4,970,567, entitled "Method and Apparatus for Detecting Infrared Radiation", issued Nov. 13, 1990 (W. L. Ahlgren et al.) discloses a monolithic structure for detecting radiation having a substrate having read out signal processing electronics integrated thereon.
It is thus an object of the invention to provide TDI apparatus and method that overcomes the problems inherent in CCD and BBD techniques.